Apparatus for synthesizing urea and method for revamping the same

ABSTRACT

It is intended to enable an apparatus for synthesizing urea and a method for revamping the same to place a heavy condenser at a relatively low position and to circumvent problems associated with letting a process fluid flow within a tube in the condenser. The present invention provides an apparatus for synthesizing urea including: a synthesis reactor for reacting NH 3  with CO 2  to obtain a urea synthesis solution containing urea, unreacted NH 3  and CO 2 , and water; a stripper for stripping the urea synthesis solution with use of at least a part of raw material CO 2  to separate a gas containing the unreacted NH 3  and CO 2 ; a vertical submerged condenser having a shell and tube structure for condensing the gas in an absorbing medium on the shell side while cooling the gas with a cooling medium passing through the tube side; and recycling means for recycling a liquid obtained from this condenser to the synthesis reactor, wherein this condenser is placed below the synthesis reactor. The present invention also provides a method for revamping an apparatus for synthesizing urea having a synthesis reactor, including installing the apparatus with the stripper, the vertical submerged condenser, and the recycling means, wherein this condenser is placed below the synthesis reactor.

TECHNICAL FIELD

The present invention relates to an apparatus for synthesizing urea thatsynthesizes urea by using ammonia and carbon dioxide as raw materials.To be more specific, the present invention relates to an apparatus usedin a stripping urea process wherein a urea synthesis solution obtainedin a synthesis reactor for synthesizing urea is stripped.

BACKGROUND ART

Techniques as described below are known as to a stripping urea process.

Patent Document 1 has described a urea process using ammonia and carbondioxide as raw materials, wherein urea synthesis is effected wholly orpartly in a combi-reactor. In this process disclosed therein, a gas froma stripper is supplied to the vertical combi-reactor and condensedwholly or partially in ammonium carbamate, which is in turn transferredfrom a scrubber zone to a condensation section via a downcomer, andammonia and carbon dioxide are partially synthesized into urea in thecondensation zone of the combi-reactor while the further conversion tourea is conducted in the reaction zone of the combi-reactor.

Patent Document 2 has described a combined reactor installation usingammonia and carbon dioxide as raw materials, which is composed of twovertically installed synthesis zones and one condensation zone. Thisinstallation is a vertical combined reactor in which two reaction zonesare separated by a high-pressure condensation zone. The document hasalso disclosed another combined reactor including two reaction zones anda high-pressure condensation zone placed outside the reactor, and hasfurther disclosed a process using this installation. Moreover, a methodinvolving wholly or partly feeding a gas supplied from a stripper to thehigh-pressure condensation zone has also been disclosed therein. In thedisclosure, it is preferred that the gas from the stripper is suppliedvia an ammonia ejector to the second reaction zone in the verticallyinstalled combined reactor.

Patent Document 3 has described an improving method capable of placingan apparatus on the ground in a synthesis method of urea including astripping step of unreacted ammonia and carbon dioxide at a pressurenearly equal to the pressure of urea synthesis by raw material carbondioxide and a condensation step of a gas mixture from the strippingstep. In this synthesis method of urea disclosed therein, a verticalcondenser for bringing a gas mixture from a stripper into contact withan absorbing medium under cooling and thereby condensing the gas mixtureis provided above a urea synthesizing tower, and a first down pipe forcommunicating the top of the condenser to the bottom of the synthesizingtower is provided, whereby the produced condensed liquid is made to flowto the bottom of the synthesizing tower by gravity and then subjected tourea synthesis together with a part of raw material ammonia or carbondioxide fed thereto, and the produced urea synthesis solution isintroduced through a second down pipe having an opening in the top ofthe synthesizing tower into the stripper by gravity, in which unreactedammonia and carbon dioxide are then separated as the gas mixture by theremainder of raw material carbon dioxide, then introduced to the bottomof the condenser, and condensed, or alternatively, a condensed liquidfrom the vertical condenser is sucked by an ejector using preheated rawmaterial ammonia as a driving fluid, then introduced into the bottom ofthe urea synthesizing tower, and subjected to urea synthesis.

Patent Document 4 has described a method for synthesizing urea with asmall volume of necessary equipment per unit production amount, whereinthe condensation of a gas mixture of unreacted ammonia and carbondioxide and the synthesis of urea are performed in a single vessel.

In the disclosure, this method for synthesizing urea includes: feeding agas mixture obtained by stripping unreacted ammonia and unreacted carbondioxide with use of raw material carbon dioxide and an absorbing mediumto the bottom of a vertical condensation synthesizing tower; feeding rawmaterial liquid ammonia to the bottom and middle of the verticalcondensation synthesizing tower; condensing the gas mixture by coolingthe part ranging from the bottom to the middle of the verticalcondensation synthesizing tower while carrying out the synthesis ofurea; introducing the generated urea synthesis solution into the top ofa stripper from the top of the vertical condensation synthesizing tower;and subjecting unreacted ammonia and carbon dioxide in the ureasynthesis solution to stripping with use of raw material carbon dioxide.

Patent Document 5 has disclosed that the height of an apparatus for asynthesis step can be reduced drastically in a CO₂ stripping ureaprocess by placing a synthesis reactor horizontally.

Patent Documents 6 and 7 have disclosed a process wherein a ureasynthesis solution coming out of a horizontal submerged condenser isintroduced into a synthesis reactor with use of an ejector.

Patent Document 1: International Publication WO 00/43358 Patent Document2: International Publication WO 01/72700 Patent Document 3: JapanesePatent Laid-Open No. 10-182587 Patent Document 4: Japanese PatentLaid-Open No. 2002-20360 Patent Document 5: Japanese Patent Laid-OpenNo. 59-122452 Patent Document 6: Japanese Patent Laid-Open No. 11-180942Patent Document 7: International Publication WO 00/00466 DISCLOSURE OFTHE INVENTION Problems to be Solved by the Invention

A conventional vertical submerged condenser for condensing a gas from astripper has been provided above a synthesis reactor for synthesizingurea in the vertical direction.

However, the vertical submerged condenser has cooling pipes and furthera tubesheet fixing the cooling pipes thereon and is therefore heavy. Theinstallation of the condenser above the synthesis reactor means theinstallation of the condenser at a relatively high position. When such aheavy article is installed at a high position, the equipmentinstallation and fixation thereof are not easy, and cost thereof tendsto rise.

In FIG. 2 of Patent Document 1, a vertical combi-reactor having acondensation zone is placed at a position lower than that of a reactorin the sheet of the drawing. However, in the vertical combi-reactorshown therein, a process fluid is made to flow on the tube side.Therefore, the residence time thereof is short. Thus, since thegeneration of water by urea synthesis is substantially absent within thetube, resulting in no absorption effect of ammonia and carbon dioxidegases, the vapor pressure-reducing effect attributed to the generationof urea, of which vapor pressure is zero, cannot be expected. Moreover,the vertical combi-reactor also presents problems such as the difficultyin evenly dispersing the process fluid to each tube and an increase inweight and in cost due to two thick tubesheets necessary for enduring ahigh pressure exceeding 13 MPa.

Patent Document 4 shows a vertical condensation synthesizing towerhaving a condensation section in a lower part and a synthesis section inan upper part. However, the condensation section and the synthesissection shown therein are not divided from each other and are in aregion in a single vessel. Thus, this equipment must be placed at a highposition or installed with a pump or a blower for supplying a synthesissolution from this region to a stripper.

An object of the present invention is to provide an apparatus forsynthesizing urea, which is capable of installing a heavy condenser at arelatively low position and can circumvent problems associated withletting a process fluid flow within a tube in the condenser.

Another object of the present invention is to provide a revampingmethod, which can revamp an existing apparatus for synthesizing urea tothereby obtain the apparatus for synthesizing urea as described above.

Means for Solving the Problems

The present invention is as described below.

1) An apparatus for synthesizing urea comprising:

a synthesis reactor for reacting ammonia with carbon dioxide to obtain aurea synthesis solution containing urea, unreacted ammonia, unreactedcarbon dioxide and water;

a stripper for stripping the urea synthesis solution with use of atleast a part of raw material carbon dioxide to separate a gas mixturecontaining the unreacted ammonia and the unreacted carbon dioxide;

a vertical submerged condenser having a shell and tube structure forcondensing the gas mixture in an absorbing medium on the shell sidewhile cooling the gas mixture with a cooling medium passing through thetube side; and

recycling means for recycling a liquid obtained from the verticalsubmerged condenser to the synthesis reactor,

wherein the vertical submerged condenser is placed below the synthesisreactor.

2) The apparatus according to 1), wherein the recycling means comprisesan ejector of which driving source is raw material ammonia.

3) The apparatus according to 1) or 2), wherein the synthesis reactorand the vertical submerged condenser are divided from each other andintegrated with each other, and the synthesis reactor is placed abovethe vertical submerged condenser.

4) The apparatus according to 3), further comprising a scrubber forscrubbing a gas that has not been condensed in the vertical submergedcondenser, wherein the scrubber is placed in the interior of thevertical submerged condenser.

5) The apparatus according to 1) or 2), further comprising a scrubberfor scrubbing a gas that has not been condensed in the verticalsubmerged condenser, wherein the vertical submerged condenser and thescrubber are integrated with each other.

6) The apparatus according to 5), wherein the scrubber is placed in theinterior of the vertical submerged condenser.

7) The apparatus according to 5) or 6), wherein the synthesis reactor isa horizontal type.

8) A method for revamping an existing apparatus for synthesizing ureahaving a synthesis reactor for reacting ammonia with carbon dioxide toobtain a urea synthesis solution containing urea, unreacted ammonia,unreacted carbon dioxide and water, comprising installing the apparatuswith:

a stripper for stripping the urea synthesis solution with use of atleast a part of raw material carbon dioxide to separate a gas mixturecontaining the unreacted ammonia and the unreacted carbon dioxide;

a vertical submerged condenser having a shell and tube structure forcondensing the gas mixture in an absorbing medium on the shell sidewhile cooling the gas mixture with a cooling medium passing through thetube side; and

recycling means for recycling a liquid obtained from the verticalsubmerged condenser to the synthesis reactor,

wherein the vertical submerged condenser is placed below the synthesisreactor.

9) A method for revamping an existing apparatus for synthesizing ureahaving:

a synthesis reactor for reacting ammonia with carbon dioxide to obtain aurea synthesis solution containing urea, unreacted ammonia, unreactedcarbon dioxide and water;

a stripper for stripping the urea synthesis solution with use of atleast a part of raw material carbon dioxide to separate a gas mixturecontaining the unreacted ammonia and the unreacted carbon dioxide; and

a vertical falling liquid film condenser for condensing the gas mixture,comprising installing the apparatus with:

a vertical submerged condenser having a shell and tube structure forcondensing the gas mixture in an absorbing medium on the shell sidewhile cooling the gas mixture with a cooling medium passing through thetube side; and

recycling means for recycling a liquid obtained from the verticalsubmerged condenser to the synthesis reactor,

wherein the vertical submerged condenser is placed below the synthesisreactor.

10) The revamping method according to 9), wherein the vertical fallingliquid film condenser includes cooling means, and

the method further comprises providing the apparatus with a line fordirecting a liquid at the outlet of the stripper to the cooling means.

ADVANTAGES OF THE INVENTION

According to the present invention, an apparatus for synthesizing ureais provided, which is capable of installing a heavy condenser at arelatively low position and can circumvent problems associated withletting a process fluid flow within a tube in the condenser.

A revamping method is also provided, which can revamp an existingapparatus for synthesizing urea to thereby obtain the apparatus forsynthesizing urea as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing one embodiment of an apparatus forsynthesizing urea of the present invention;

FIG. 2 is a flow diagram showing an alternative embodiment of theapparatus for synthesizing urea of the present invention;

FIG. 3 is a flow diagram showing a further alternative embodiment of theapparatus for synthesizing urea of the present invention;

FIG. 4 is a flow diagram showing a further alternative embodiment of theapparatus for synthesizing urea of the present invention;

FIG. 5 is a flow diagram showing a further alternative embodiment of theapparatus for synthesizing urea of the present invention; and

FIG. 6 is a flow diagram for illustrating a urea production plant.

DESCRIPTION OF SYMBOLS

-   A: synthesizing tower-   B: condenser-   C: stripper-   D: scrubber-   E: ejector-   F: gas-liquid separator-   G: ammonia preheater-   H: carbon dioxide compressor-   I: vertical falling liquid film condenser-   1: raw material ammonia-   2: raw material carbon dioxide-   3: synthesis solution at condenser outlet-   4: synthesis solution at synthesis reactor outlet-   5: outlet gas at stripper top-   6: outlet liquid at gas-liquid separator-   7: outlet gas at gas-liquid separator-   8: liquid at ejector outlet-   9: liquid at scrubber outlet-   10: liquid at stripper outlet-   11: recycled carbamate liquid-   12: gas at scrubber outlet-   13: cooling medium at tube inlet of condenser (boiler water)-   14: cooling medium at tube outlet of condenser (boiler water and    steam)-   15: steam for heating stripper shell-   16: condensed water at outlet of stripper shell-   21: gas containing ammonia and carbon dioxide obtained from    decomposition apparatus-   22: urea solution obtained from the decomposition apparatus-   23: molten urea-   24: granular urea

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described with reference todrawings. However, the present invention is not intended to be limitedto them. The positional relationships of component parts of an apparatusin the vertical direction are shown in FIGS. 1 to 5.

Hereinafter, a vertical submerged condenser is simply referred to as acondenser in some cases.

The vertical submerged condenser is a condenser having a vertical shelland tube heat exchanger structure wherein a cooling pipe is completelyimmersed in a liquid phase on the shell side.

FIG. 1 is a flow diagram showing one embodiment of an apparatus forsynthesizing urea of the present invention, which is suitable for a CO₂stripping urea producing process. This apparatus includes a synthesisreactor A including a synthesis zone, a condenser B including acondensation zone, a stripper C for treating unreacted components in asynthesis solution 4 at the outlet of the synthesis reactor, a scrubberD for absorbing uncondensed gases from the condenser into an absorbingmedium, and an ejector E for pressurizing.

In this contest, the synthesis zone means a region wherein of thereactions of the formulas 1 and 2 described later, mainly the reactionof the formula 2 progresses, and the condensation zone means a regionfor condensing an ammonia gas and/or a carbon dioxide gas into theabsorbing medium, wherein the reaction of the formula 1 as well as thereaction of the formula 2, which is the dehydration reaction of ammoniumcarbamate formed by the reaction of the formula 1, progresses.

These component parts may be installed separately or may be installed,if desired, in combination such as the synthesis reactor and thecondenser or the condenser and the scrubber. Moreover, the synthesisreactor may be a vertical or horizontal type, which is appropriatelydetermined from the viewpoint of cost efficiency and so on.

In the present invention, the condenser is placed below the synthesisreactor. Specifically, the upper end of the condenser is provided lowerthan the upper end of the synthesis reactor in the vertical direction.

In the condensation zone and the synthesis zone, ammonium carbamate(hereinafter, referred to as carbamate in some cases) is formed by thereaction of ammonia and carbon dioxide, and the formed carbamate isdehydrated to thereby form urea, as shown in the formulas 1 and 2described below. The reaction rate of carbamate formation is high, andthe urea formation reaction by the dehydration of carbamate is anequilibrium reaction.

2NH₃+CO₂→NH₂CO₂NH₄ (Exothermic reaction)  Formula 1

NH₂CO₂NH₄

CO(NH₂)₂+H₂O (Endothermic reaction)  Formula 2

Raw material liquid ammonia 1 is pressurized to a desired pressure by anammonia pump (not shown), and a part 1 a thereof is heated by a heatexchanger G and supplied to the ejector E. A urea synthesis solution 6from the condenser B is supplied to the ejector and pressurized. On theother hand, raw material gaseous carbon dioxide 2 is pressurized to adesired pressure by a carbon dioxide compressor H, and the greater part2 a thereof is supplied to the stripper C. The remaining portion 2 b ofthe carbon dioxide is supplied to the synthesis reactor A for thepurpose of controlling the temperature of the synthesis reactor andsupplying oxygen for corrosion prevention. Air for corrosion prevention(not shown) is usually supplied to the first stage suction side or tothe middle stage of the carbon dioxide compressor. The heat exchanger Gcan appropriately adopt a heat exchanger known in the art having astructure that can heat raw material ammonia. A fluid having a desiredtemperature level can be used appropriately as a heat medium for heatingin the heat exchanger G.

A small portion of the whole ammonia raw material may be supplied, tothe condenser, as raw material ammonia 1 b for the corrosion preventionof welds of a condenser tubesheet. The feed line for this raw materialammonia 1 b is not always necessary.

Although carbon dioxide 2 c is used as a stripping agent in a downstreamdecomposition step, the line for this carbon dioxide 2 c is not alwaysnecessary.

An ammonia-containing synthesis solution 8 from the ejector E issupplied to the synthesis reactor, as with carbon dioxide.

This ejector constitutes recycling means for recycling the liquid 6obtained from the condenser B to the synthesis reactor A. In FIG. 1, therecycling means includes a gas-liquid separator F, the ejector E, andthe lines 3, 6, and 8. The line 6 is provided with a regulating valvefor the liquid level regulation of the gas-liquid separator F. Althoughalternative pressurizing means such as a pump may be used instead of theejector for recycling, the ejector is preferable because of its simplestructure and excellent endurance and maintainability. An operatingcondition (pressure difference between the lines 6 and 8) for theejector may be set to, for example, not less than 0.2 Mpa and not morethan 1 MPa.

In the synthesis zone in the interior of the synthesis reactor, the ureasynthesizing reaction proceeds, preferably close to an equilibriumsynthesis rate, to thereby synthesize urea. In the interior of thesynthesis reactor, it is preferred that urea is synthesized at apressure of not less than 13 MPaG and not more than 25 MPaG (G in thepressure unit denotes a gage pressure), at a temperature of not lessthan 170° C. and not more than 210° C., at a mole ratio of ammonia(including ammonia that has been converted to ammonium carbamate andurea) to carbon dioxide (including carbon dioxide that has beenconverted to ammonium carbamate and urea) (hereinafter, referred to asN/C) of not less than 3.0 and not more than 4.5, at a mole ratio ofwater (excluding water formed by the urea synthesizing reaction) tocarbon dioxide (including carbon dioxide that has been converted toammonium carbamate and urea) (hereinafter, referred to as H/C) of notmore than 1.0, and with a residence time of not less than 10 minutes andnot more than 40 minutes.

The synthesis temperature can be controlled, for example, by thepreheating temperature of the ammonia 1 a driving the ejector and/or bythe amount of the carbon dioxide 2 b supplied to the synthesis reactor.N/C can be determined, for example, by continuously measuring thedensity of the liquid 4 at the outlet of the synthesis reactor with adensity meter or by regularly sampling and quantitatively analyzing theliquid 4 at the outlet of the synthesis reactor. N/C can be adjusted,for example, by adjusting the amount of the ammonia 1 a supplied to theejector. H/C is often determined depending on the amount of waternecessary for the absorption of unreacted substances (ammonia and carbondioxide) in a recovery apparatus (not shown in FIG. 1) for recoveringthe unreacted substances discharged from the synthesis reactor. Sincewater inhibits the urea synthesizing reaction in terms of equilibrium(lower H/C is better in terms of synthesis equilibrium), it is preferredthat the amount of water supplied to this recovery apparatus is arequisite minimum. The recovery apparatus will be described later.

In the synthesis reactor, the urea synthesis rate with respect to carbondioxide is determined depending on chemical equilibrium and is on theorder of not less than 60% and not more than 75% when the N/C range isnot less than 3.0 and not more than 4.5.

The synthesis rate with respect to carbon dioxide is the ratio of thenumber of moles of the carbon dioxide that has been converted to ureaamong the supplied carbon dioxide to the number of moles of the carbondioxide supplied to the equipment or the region to be considered, and isusually indicated in %.

The synthesis pressure set to 13 MPaG or more is preferable from theviewpoint of allowing for the adoption of an operating pressure having amargin for the synthesis equilibrium pressure at a temperature (170° C.or higher) preferable for urea synthesis and from the viewpoint ofpreventing a decrease in the synthesis rate attributed to vaporization.The synthesis pressure set to 25 MPaG or less is preferable from theviewpoint of allowing for reduction in energy for pressurizing the rawmaterial ammonia, the raw material carbon dioxide, and the unreactedcarbamate liquid 6 and from the viewpoint of equipment cost.

The carbamate liquid refers to a liquid obtained by recovering theunreacted ammonia and carbon dioxide as an aqueous solution of ammoniumcarbamate in the recovery step downstream from the synthesis step.

The synthesis temperature set to 170° C. or higher is preferable fromthe viewpoint of preventing a decrease in the reaction rate of ureaformation. Moreover, the synthesis temperature set to 210° C. or loweris preferable from the viewpoint of preventing a rise in the risk ofso-called active corrosion in addition to an increase in a corrosionrate.

N/C is preferably 3.0 or more from the viewpoint of the equilibriumsynthesis rate and is preferably 4.5 or less from the viewpoint ofpreventing a gaseous phase from being easily generated due to anincreased ammonia vapor pressure.

The stoichiometric ratio of ammonia to carbon dioxide (N/C) in ureasynthesis is 2 as shown in the formulas 1 and 2 described above.Actually, the state where excessive unreacted ammonia is present as aresult of excessive ammonia supply is preferable for increasing the ureaequilibrium synthesis rate.

H/C is preferably 1.0 or less, more preferably 0.7 or less, from theviewpoint of the urea synthesis rate. H/C may be zero. Actually, wateroften exist to a certain extent because H/C is often determineddepending on the amount of water necessary for the absorption ofunreacted substances (ammonia and carbon dioxide) in a recoveryapparatus (not shown in FIG. 1) for recovering the unreacted substancescoming out of the apparatus for synthesizing urea. H/C may be set to,for example, 0.4 or more.

The residence time of the process fluid in the synthesis reactor set to10 minutes or more is preferable from the viewpoint of causing theprogress of the urea synthesizing reaction. The residence time ispreferably 40 minutes or less because, even if the residence timeexceeds 40 minutes, the synthesis rate has already reached nearly theequilibrium synthesis rate and cannot be practically expected to furtherrise.

Urea is synthesized in the condensation zone within the condenser B andin the synthesis zone within the synthesis reactor A. A urea-containingeffluent substance 4 coming out of the synthesis reactor is supplied tothe stripper C. The effluent substance 4 at the outlet of the synthesisreactor contains the synthesized urea, water, carbamate, and unreactedammonia as liquid phases and a portion of unreacted ammonia and carbondioxide as gaseous phases together with inert gases.

In this context, the inert gases are a general term for air introducedfor preventing the corrosion of the apparatus for synthesizing ureacomposed of, for example, the synthesis reactor, the stripper, thecondenser, the scrubber, and the piping connecting them and so on, andfor impurities such as hydrogen and nitrogen contained in the rawmaterial carbon dioxide.

The effluent substance 4 from the synthesis reactor, after beingsupplied to the stripper C, is heated by steam for heating, and thecarbamate contained in the effluent substance from the synthesis reactoris thereby thermally decomposed. Moreover, the unreacted ammonia and theunreacted carbon dioxide in the effluent substance from the synthesisreactor are CO₂-stripped by the supplied raw material carbon dioxide 2 aand divided into gaseous components 5 containing carbon dioxide, ammoniaand inert gases and into a synthesis solution 10. This synthesissolution usually has a urea concentration of not less than 40% by massand not more than 60% by mass inclusive.

The stripping refers to an operation wherein a component dissolved in asolution is released from the liquid by heating and/or by contact with astripping agent (usually, a gas that is insoluble or poorly soluble inthe solution) and separated as a gaseous phase.

The stripper C has a shell and tube heat exchange structure, wherein onthe shell side, the steam 15 for heating is supplied, and condensedwater 16, a condensate of this steam, is discharged. The effluentsubstance from the synthesis reactor is heated when passing through thetube side. As described above, the stripping performed both by heatingand by carbon dioxide is preferable because not only excellent strippingeffect but also carbamate-decomposing effect is obtained. Since thestripper C also has the function of gas-liquid separation, it is notnecessary to additionally provide a gas-liquid separator for separatingthe effluent substance 4 from synthesis reactor into gas and liquid.

The gaseous components 5 from the stripper are supplied to thecondenser. For this purpose, the stripper is connected to the shell sideof the condenser. On the other hand, the synthesis solution 10 from thestripper is sent to a decomposition apparatus (not shown in FIG. 1), inwhich the urea component thereof is further purified.

An absorbing medium is supplied to the scrubber D. Recycled carbamateliquid 11 recovered in the decomposition apparatus and in the recoveryapparatus (not shown in FIG. 1) is used as this absorbing medium. Thedecomposition apparatus, the recovery apparatus, and the recycledcarbamate liquid will be described later.

Here, the recycled carbamate liquid 11 is supplied as the absorbingmedium to the scrubber D and supplied from the line 9 to the condenserafter absorbing a portion of ammonia and carbon dioxide gas contained inthe gaseous components 7 by contact with the gaseous components 7 fromthe condenser B. Ammonia, carbon dioxide gas, and inert gases that havenot been absorbed into the recycled carbamate liquid 11 are sent fromthe line 12 to the recovery apparatus.

The scrubbing refers to an operation wherein a certain component in gasis absorbed into liquid by contact between the gas and the liquid tothereby clean the gas.

It is preferred that the condenser is operated at a pressure of not lessthan 13 MPaG and not more than 25 MPaG, at a temperature of not lessthan 160° C. and not more than 200° C., at N/C of not less than 2.5 andnot more than 4.0, at H/C of not more than 1.0, and with a residencetime of not less than 10 minutes and not more than 30 minutes.

The N/C of the condenser is secondarily determined depending on the N/Cof the synthesis reactor. Specifically, the composition of the gas 5 atthe outlet of the stripper is determined mainly depending on the N/C ofthe synthesis reactor, with the result that the N/C of the condenser wasalso determined. The H/C of the condenser is determined depending on theamount of water necessary for the absorption of unreacted substances(ammonia and carbon dioxide) in a recovery apparatus for recovering theunreacted substances coming out of the apparatus for synthesizing urea.Since water inhibits the urea synthesizing reaction in terms ofequilibrium (lower H/C is better in terms of synthesis equilibrium), itis preferred that the amount of water supplied to this recoveryapparatus is a requisite minimum.

The condensation section and the stripper are operated at substantiallythe same pressure as that for the synthesis reactor.

The temperature of the process fluid within the condenser is preferably160° C. or higher from the viewpoint of the reaction rate of ureaformation and is preferably 200° C. or lower from the viewpoint ofsuppressing both a decrease in the condensation rate with an increase inthe vapor pressure and the corrosion of the equipment materials.

The N/C of the process fluid within the condenser is preferably 2.5 ormore from the viewpoint of suppressing a decrease in the condensationrate attributed to an increase in the partial pressure of carbon dioxidein the urea synthesis solution and is preferably 4.0 or less from theviewpoint of suppressing a decrease in the condensation rate resultingfrom an increase in the vapor pressure of ammonia.

H/C is preferably 1.0 or less from the viewpoint of the urea synthesisrate.

The residence time within the condenser is preferably 10 minutes or morefrom the viewpoint of suppressing both an increase in the vapor pressureattributed to a decrease in the urea synthesis rate and a decrease inthe condensation rate. The residence time is preferably 30 minutes orless because, even if the residence time exceeds 30 minutes, the ureasynthesis rate cannot be expected to remarkably rise.

The carbamate liquid 9, which has absorbed a portion of the gaseouscomponents 7 supplied from the stripper C through the condenser B andthe gas-liquid separator F to the scrubber D, is supplied to thecondenser. In the condenser, the carbamate liquid 9 and gaseouscomponents 5 contact with each other, and ammonia and carbon dioxide areabsorbed into the carbamate liquid and condensed, followed by thecarbamate forming reaction represented by the formula 1 and thecarbamate dehydration reaction represented by the formula 2 to therebyform urea.

The conversion ratio with respect to carbon dioxide in the condenser is,for example, not less than 20% and not more than 60%.

Ammonia and carbon dioxide that have not been condensed in thecondensation zone are separated together with inert gases in the topsection of the condenser or in the gas-liquid separator and sent to therecovery apparatus or to the scrubber. Here, a gas mixture of them isseparated from the liquid in the gas-liquid separator F and sent to thescrubber D.

The liquid 6 obtained from the gas-liquid separation of the fluid 3 atthe outlet of the condenser is supplied to the ejector E, then sent tothe synthesis reactor where raw material ammonia is used as a drivingsource, and subjected to the further urea synthesizing reaction.

Hereinafter, each component part of the apparatus will be described indetail.

The synthesis reactor A may be a vertical- or horizontal-type reactor,the interior of which is installed with baffle plates, a gasdistributor, and so on. Moreover, the synthesis reactor may adopt, ifdesired, a structure wherein the synthesis reactor is constructedintegrally with the condenser and so on. However, the synthesis reactorand the condenser, even when constructed integrally, are divided fromeach other by a partition plate or the like and constructedindependently.

The stripper can appropriately adopt a structure that is capable ofperforming gas-liquid contact and/or a structure that can achieve thedecomposition of carbamate in the synthesis solution 4 and the emissionof dissolved gases by heating.

The stripper may have a vertical shell and tube heat exchange structure,for example, as shown in FIG. 1. In this case, a heat medium such assteam and the synthesis solution 4 are supplied to the shell side and tothe tube side, respectively, and heat may be supplied from the shellside to the tube side. Other than such a structure, a plate tower and apacked tower may also be used as the stripper. A combination thereof mayalso be adopted.

The structure of the scrubber can appropriately adopt a structure thatis capable of scrubbing and may adopt a packed tower packed withpacking, a shell and tube structure, a plate tower, and a combinationthereof. The scrubber is operated at substantially the same pressure asthat for the synthesis reactor and usually at a temperature of not lessthan 100° C. and not more than 180° C. In the scrubber, absorption heatis generated by the absorption of gases. The scrubber having the shelland tube structure allows for the removal and recovery of the absorptionheat by a cooling medium.

The scrubber may be built in the condenser or may be constructedintegrally with the condenser, as appropriate.

The condenser can adopt, as appropriate, a vertical submerged condenserhaving a structure that can condense the gaseous components, absorbammonia and carbon dioxide, and bring about the urea synthesizingreaction represented by the reaction formulas 1 and 2. The vertical typehas an advantage in easily distributing gas evenly within the condenserand easily securing a small installation area and a long residence timeof gas. Since the urea synthesizing reaction occurs in a liquid phase,it is desirable to insert a cooling pipe into the liquid. Therefore, avertical submerged condenser is used.

The condenser may adopt, for example, as shown in FIG. 1, a structurewherein a U tube is installed as cooling means. This structure issuitable for the submerged condenser. That is, the U tube is easilybrought into the state where it is completely immersed in the liquidphase. A cooling medium such as boiler water may be supplied to the tubeside. Alternatively, a process fluid such as liquid ammonia and a urealiquid may be allowed to flow as a cooling medium to thereby perform thecooling of the fluid on the shell side simultaneously with thepreheating or heating of the process.

If the U tube is used, only one tubesheet suffices. This structure iseffective for weight saving.

Moreover, the condenser and the synthesis reactor may be constructedintegrally, if desired.

An example of the condenser constructed integrally with the verticalsynthesis reactor is shown in FIG. 1. The condenser is installed belowthe synthesis reactor so that the condenser is integrated with the lowerpart of the synthesis reactor via a partition plate.

The pressurized raw material ammonia 1 a and the liquid 6 obtained fromthe condenser are supplied to the ejector E, and the mixed fluid 8 ofthem is supplied to the lower part of the synthesis reactor A. A portion2 b of the pressurized raw material carbon dioxide containing air(supplied from, e.g., the intermediate stage of the pressurizing meansH) is supplied to the synthesis reactor. These supplied raw materialsascend in the synthesis reactor, during which reactions according to thereaction formulas 1 and 2 occur in the synthesis reactor to thereby formurea and so on. It is preferred that baffle plates for promoting mixingand reactions are installed within the synthesis reactor. The synthesissolution in which the reactions have preferably reached nearlyequilibrium passes through downward the down pipe having an inletinstalled in the upper part of the synthesis reactor and is suppliedfrom the line 4 to the upper part of the stripper. The pressurized rawmaterial carbon dioxide 2 a is supplied from the lower part of thestripper and strips unreacted ammonia in the synthesis solution 4 anddecomposition products of carbamate.

The stripper shown in FIG. 1 has a vertical shell and tube heat exchangestructure. On the tube side, the unreacted substance (carbamate) in thesynthesis solution 4 and excessive ammonia are stripped as gaseouscomponents (ammonia and carbon dioxide) by countercurrent contactbetween the synthesis solution 4 from the synthesis reactor and the rawmaterial carbon dioxide 2 a. Steam is supplied to the shell side andused as a heat source for the decomposition of carbamate.

The gaseous components 5 from the upper part of the stripper aresupplied to the lower part of the condenser. The carbamate liquid 9 fromthe scrubber is also supplied to the lower part of the condenser.

A U tube is installed as cooling means within the condenser. Water(boiler water) 13 for cooling is supplied to the U tube, and a fluid 14(mixed fluid of boiler water and steam), which has been subjected to thecooling, is discharged from the U tube.

The carbamate liquid 9 supplied as an absorbing medium to the shellportion of the condenser and the gaseous components 5 from the stripperare brought into contact with each other and cooled while ascending inthe interior of the condenser. The gaseous components are condensed andabsorbed into the carbamate liquid to thereby form carbamate, followedby the further urea synthesizing reaction.

In FIG. 1, the gas-liquid separator F is installed independently. In thegas-liquid separator, the fluid 3 discharged from the condenser isdivided into gaseous components 7 and a liquid 6. The gases and theliquid are supplied to the scrubber and to the ejector, respectively.

As described above, the condenser is placed below the synthesis reactor,whereby a cooling pipe such as a U tube and a tubesheet thereof havinghuge weights can be placed at a relatively low position, therebyfacilitating equipment installation and fixation.

The condenser provided with the U tube in the upper part has a structurewherein the U tube extends downward from the tubesheet. In such a case,when a gaseous phase portion is developed in the interior of thecondenser, the gas-liquid interface comes into contact with the outersurface of the tube. That is, the tube spans both the gaseous phaseportion and the liquid phase portion. In such a situation, the tubemight be caused to be susceptible to corrosion attributed to thecondensation of carbamate. However, in the condenser shown in FIG. 1,the tubesheet is provided in the lower part, particularly the bottom, ofthe condenser, and the U tube extends upward from the tubesheet.Moreover, a zone free of the U tube can be provided in the upper part ofthe condenser. Thus, the U tube can be immersed completely in the liquideven if the gaseous phase portion exists in the interior of thecondenser. This structure is preferable because of posing no risk of thecorrosion as described above.

In the condenser constructed integrally with the synthesis reactor, theU tube has no other choice but to extend downward from the tubesheet ifthe condenser is provided above the synthesis reactor. However, evenwhen the condenser is constructed integrally with the synthesis reactor,a zone free of the U tube can be provided in the upper part of thecondenser as described above by placing the condenser below thesynthesis reactor. This structure is preferable because the corrosion asdescribed above can be prevented.

The urea synthesis solution obtained in the synthesis reactor may besubjected to gas-liquid separation in the top section of the synthesisreactor or may be supplied directly as a gas-liquid mixed phase to thestripper without providing gas-liquid separation means within thesynthesis reactor.

An example of a urea production plant having an apparatus forsynthesizing urea will be described. An outline of the urea productionplant is shown in FIG. 6. The urea production plant includes anapparatus for synthesizing urea, a recovery apparatus, a decompositionapparatus, a concentration apparatus, and a finishing apparatus. Rawmaterial ammonia 1 and raw material carbon dioxide 2 are supplied to theapparatus for synthesizing urea. A urea synthesis solution 10, which hasundergone stripping, is sent from the apparatus for synthesizing urea tothe decomposition apparatus.

In the decomposition apparatus, unreacted ammonia and carbamatecontained in the urea synthesis solution are decomposed by heating thesupplied urea synthesis solution under reduced pressure and separated asa gas 21 containing ammonia and carbon dioxide. The remaining liquidphase is sent as an aqueous solution of urea 22, for example, on theorder of 68% by mass, to the concentration apparatus downstream of thedecomposition apparatus.

In the concentration apparatus, moisture is almost completely evaporatedand separated by heating under vacuum the aqueous solution of urea 22obtained in the decomposition apparatus, to thereby obtain molten urea23, for example, on the order of 99.7% by mass. This molten urea is sentto the finishing apparatus downstream of the concentration apparatus, inwhich the molten urea is then subjected to cooling and solidificationand finished as granular urea 24.

On the other hand, the gas 21 containing ammonia and carbon dioxide,which has been separated in the decomposition apparatus, and a gas 12containing ammonia, carbon dioxide, and inert gases, which has not beenabsorbed into a recycled carbamate liquid in the scrubber, are absorbedinto water in the recovery apparatus and recovered as an aqueoussolution of ammonium carbamate. This aqueous solution is pressurized andthen returned as a recycled carbamate liquid 11 to the apparatus forsynthesizing urea.

An alternative embodiment of the present invention is shown in FIG. 2.This embodiment is different from the embodiment shown in FIG. 1 in thata scrubber D is installed between a synthesis reactor A and a condenserB. A gaseous phase portion exists in the upper part of the condenser,and the scrubber is provided within this gaseous phase portion.

A recycled carbamate liquid 11 is supplied to the installed scrubber D.In this embodiment, since the scrubber is installed within the synthesisreactor, it is unnecessary to additionally place not only a scrubber butalso a gas-liquid separator. Thus, this structure is effective in termsof reduction in high-pressure piping, in equipment cost, and inconstruction cost. The gas-liquid separation can be performed byproviding the gaseous phase portion in the upper part within thecondenser B. Gaseous components 7 ascend from the gas-liquid interfaceand are then cleaned in the scrubber D and sent to a line 12, whereasliquid components 6 are sent to an ejector E through a down pipe havingan inlet below the gas-liquid interface. A carbamate liquid 9, which hasabsorbed a portion of the gaseous components 7, is supplied through thedown pipe to the lower part of the condenser.

A further alternative embodiment of the present invention is shown inFIG. 3. In this embodiment, a scrubber D is built in a condenser B, anda synthesis reactor A is separately provided in isolation. As with theembodiment of FIG. 2, the scrubber is provided in a gaseous phaseportion in the upper part of the condenser. Since gas-liquid separationis performed in the gaseous phase portion provided in the upper part ofthe condenser, no gas-liquid separator is required.

In this embodiment, the condenser and the synthesis reactor areconstructed separately, thereby facilitating equipment manufacture andtransport and works such as installation work. Moreover, this embodimentis suitable for revamping an apparatus for synthesizing urea in anold-type plant having an already-existing synthesis reactor, forexample, a solution recycling (non-stripping) apparatus for synthesizingurea. By adding a stripper C, a condenser B, and recycling means forrecycling a liquid obtained from the condenser to the synthesis reactorand so on to the plant having an already-existing synthesis reactor,wherein the condenser B is placed below the synthesis reactor A in thevertical direction to obtain this embodiment, it is possible to achieveenhancement in installed capacity and improvement in efficiency.

A further alternative embodiment of the present invention is shown inFIG. 4. This embodiment is modified from the embodiment shown in FIG.103, by placing a synthesis reactor A horizontally. The use of thehorizontally placed synthesis reactor can make the equipment height(particularly, at the top of the synthesis reactor) small and iseffective for reduction in equipment cost.

An existing stripping-type apparatus for synthesizing urea having asynthesis reactor, a stripper, and a vertical falling liquid filmcondenser may be revamped by installing this existing synthesizingapparatus with a vertical submerged condenser and recycling means forrecycling a liquid obtained from the vertical submerged condenser to thesynthesis reactor. An embodiment shown in, for example, FIG. 5 issuitable for boosting the production of urea by adding a verticalsubmerged condenser B and an ejector E as well as appropriate piping toan already-existing CO₂ stripping-type apparatus for synthesizing ureaincluding a scrubber D additionally installed at a position higher thanthat of a synthesis reactor A and both a vertical falling liquid filmcondenser I and a synthesis reactor A installed at a position higherthan that of a stripper C.

An effluent substance 19 (containing a gas mixture that has beenseparated in the stripper and has not been condensed in the condenser I)at the outlet of the vertical falling liquid film condenser I issupplied to the vertical submerged condenser B. For this purpose, theshell side of the vertical submerged, condenser is connected to thevertical falling liquid film condenser. A gas 20 at the outlet of thesynthesis reactor is scrubbed in the scrubber D.

The vertical falling liquid film condenser I also includes cooling meansfor condensation. For example, boiler water 17 is supplied to the shellside of the condenser I and heated, and boiler water 18 partiallyconverted to steam is discharged from the shell.

Alternatively, a liquid 10 at the outlet of the stripper may also beused instead of the boiler water 17 as a cooling medium for cooling thevertical falling liquid film condenser I. This is efficient because theheating of the liquid at the outlet of the stripper in the decompositionstep performed downstream of the apparatus for synthesizing urea canpartially be performed along with the cooling of the vertical fallingliquid film condenser I. For example, a line for directing the liquid atthe outlet of the stripper to the shell side of the condenser I may beprovided, and the heated liquid at the outlet of the stripper dischargedfrom the shell side may be utilized in the downstream decompositionstep.

EXAMPLES

Hereinafter, the present invention will be described more specificallywith reference to Examples. However, the present invention is notintended to be limited to them.

Example 1

Table 1 shows a material balance, temperatures, and pressures in anexample of urea production at 1725 t/day (t denotes a ton, i.e., 103 kg)with use of an apparatus for synthesizing urea having an embodimentshown in FIG. 1.

Of raw material ammonia 1 at a pressure of 23 MPaG and a temperature of30° C., 39.7 t/hour were heated in a heat exchanger G to 140° C. andsupplied from a line 1 a to an ejector E, while 1.0 t/hour was suppliedthrough a line 1 b to a condenser B. On the other hand, carbon dioxide 2at a pressure of 0.1 MPaG and a temperature of 40° C. was pressurized ina compressor H and thereby converted to carbon dioxide at a pressure of16 MPaG and a temperature of 120° C. And 41.2 t/hour and 9.1 t/hour ofthis carbon dioxide were supplied through a line 2 a to a stripper C andthrough a line 2 b to a synthesis reactor A, respectively, to produceurea.

The supplied ammonia 1 a in the ejector E was mixed with a synthesissolution 6 at a pressure of 15.2 MPaG from a gas-liquid separator F andsupplied at a pressure of 15.5 MPaG from a line 8 to the synthesisreactor A having a synthesis zone.

The synthesis reactor A was operated at a pressure of 15.5 MPaG, at atemperature of 182° C., at N/C of 3.7, at H/C of 0.58, and with aresidence time of 20 minutes to synthesize urea. The conversion ratiowith respect to carbon dioxide in the synthesis reactor was 63%.

A urea-containing synthesis solution 4 from a down pipe located in theupper part of the synthesis reactor A was supplied to a stripper C.Medium-pressure steam 15 was supplied to the shell side of the stripperC and discharged therefrom as condensed water 16 after supplying heatfor the decomposition of carbamate. On the tube side of the stripper C,the decomposition of carbamate and stripping were performed at an upperpart temperature of 184° C. and lower part temperature of 171° C. and ata pressure of 15.5 MPaG to thereby separate gaseous components in theupper part. The gaseous components 5 were sent to the condenser B, and asynthesis solution 10 coming out of the bottom of the stripper C wassent to a decomposition apparatus.

The gaseous components at 134.6 t/hour (line 5) from the stripper C andthe recycled carbamate liquid at 62.2 t/hour (line 11) were sent to thecondenser. The condenser was operated at a temperature of 180° C., at apressure of 15.2 MPaG, at N/C of 2.9, at H/C of 0.65, and with aresidence time of 20 minutes to synthesize urea and so on. Theconversion ratio with respect to carbon dioxide in the condenser was46%.

Example 2

The same study as in Example 1 was conducted except that an apparatusfor synthesizing urea having an embodiment shown in FIG. 2 was used.Results such as a material balance were obtained as shown in Table 1, aswith Example 1.

Example 3

The same study as in Example 1 was conducted except that an apparatusfor synthesizing urea having an embodiment shown in FIG. 3 was used.Results such as a material balance were obtained as shown in Table 1, aswith Example 1.

Example 4

The same study as in Example 1 was conducted except that an apparatusfor synthesizing urea having an embodiment shown in FIG. 4 was used.Results such as a material balance were obtained as shown in Table 1, aswith Example 1.

Example 5

A study was conducted on an example of urea production at 1725 tons/daywith use of an apparatus for synthesizing urea having an embodimentshown in FIG. 5. In the embodiment shown in FIG. 5, gaseous componentscoming out of a stripper C were partially condensed in a vertical shelland tube falling liquid film condenser I and then sent to a condenser B.Boiler water 17 was supplied to the shell side of the vertical fallingliquid film condenser, in which the boiler water 17 was partiallyconverted by heat recovery to low-pressure steam and discharged as amixed-phase flow of the boiler water and the low-pressure steam from aline 18. Supplying conditions for raw material ammonia and raw materialcarbon dioxide and operating conditions for the synthesis reactor A, thecondenser B, and the stripper C were the same as in Example 1. Resultssuch as a material balance were obtained as shown in Table 1, as withExample 1.

[Table 1]

TABLE 1 Line No. 1 1a 1b 2 2a 2b 2c 4 5 6 8 10 11 12 Urea t/h 77.3 50.850.8 73.5 0.3 Ammonia t/h 40.7 39.7 1.0 86.9 69.7 62.3 102.0 19.2 21.91.6 Carbon t/h 52.7 41.2 9.1 2.4 33.6 58.5 43.9 43.9 18.9 24.7 2.2Dioxide Water t/h 44.6 6.4 36.7 36.7 37.2 15.3 0.2 Total t/h 40.7 39.71.0 52.7 41.2 9.1 2.4 242.4 134.6 193.7 233.4 148.8 62.2 4.0 Temperature° C. 30 140 30 40 120 120 150 182 184 180 170 171 105 160 Pressure MPaG23.0 23.0 23.0 0.1 16.0 16.0 0.5 15.5 15.5 15.2 15.5 15.5 16.0 15.2 N/Cmol/mol 3.7 2.9 H/C mol/mol 0.58 0.65 CO2 conversion % 63 46 ratio

INDUSTRIAL APPLICABILITY

An apparatus for synthesizing urea of the present invention is suitablyused in urea manufacture wherein urea is manufactured from ammonia andcarbon dioxide. Moreover, a method for revamping an apparatus forsynthesizing urea of the present invention is suitable for revamping anexisting apparatus for synthesizing urea and thereby boosting productionand improving efficiency.

1. An apparatus for synthesizing urea comprising: a synthesis reactorfor reacting ammonia with carbon dioxide to obtain a urea synthesissolution containing urea, unreacted ammonia, unreacted carbon dioxideand water; a stripper for stripping the urea synthesis solution with useof at least a part of raw material carbon dioxide to separate a gasmixture containing the unreacted ammonia and the unreacted carbondioxide; a vertical submerged condenser having a shell and tubestructure for condensing the gas mixture in an absorbing medium on theshell side while cooling the gas mixture with a cooling medium passingthrough the tube side; and recycling means for recycling a liquidobtained from the vertical submerged condenser to the synthesis reactor,wherein the vertical submerged condenser is placed below the synthesisreactor.
 2. The apparatus according to claim 1, wherein the recyclingmeans comprises an ejector of which driving source is raw materialammonia.
 3. The apparatus according to claim 1, wherein the synthesisreactor and the vertical submerged condenser are divided from each otherand integrated with each other, and the synthesis reactor is placedabove the vertical submerged condenser.
 4. The apparatus according toclaim 3, further comprising a scrubber for scrubbing a gas that has notbeen condensed in the vertical submerged condenser, wherein the scrubberis placed in the interior of the vertical submerged condenser.
 5. Theapparatus according to claim 1, further comprising a scrubber forscrubbing a gas that has not been condensed in the vertical submergedcondenser, wherein the vertical submerged condenser and the scrubber areintegrated with each other.
 6. The apparatus according to claim 5,wherein the scrubber is placed in the interior of the vertical submergedcondenser.
 7. The apparatus according to claim 5, wherein the synthesisreactor is a horizontal type.
 8. A method for revamping an existingapparatus for synthesizing urea having a synthesis reactor for reactingammonia with carbon dioxide to obtain a urea synthesis solutioncontaining urea, unreacted ammonia, unreacted carbon dioxide and water,comprising: installing the apparatus with: a stripper for stripping theurea synthesis solution with use of at least a part of raw materialcarbon dioxide to separate a gas mixture containing the unreactedammonia and the unreacted carbon dioxide; a vertical submerged condenserhaving a shell and tube structure for condensing the gas mixture in anabsorbing medium on the shell side while cooling the gas mixture with acooling medium passing through the tube side; and recycling means forrecycling a liquid obtained from the vertical submerged condenser to thesynthesis reactor, wherein the vertical submerged condenser is placedbelow the synthesis reactor.
 9. A method for revamping an existingapparatus for synthesizing urea having a synthesis reactor for reactingammonia with carbon dioxide to obtain a urea synthesis solutioncontaining urea, unreacted ammonia, unreacted carbon dioxide and water;a stripper for stripping the urea synthesis solution with use of atleast a part of raw material carbon dioxide to separate a gas mixturecontaining the unreacted ammonia and the unreacted carbon dioxide; and avertical falling liquid film condenser for condensing the gas mixture;comprising installing the apparatus with: a vertical submerged condenserhaving a shell and tube structure for condensing the gas mixture in anabsorbing medium on the shell side while cooling the gas mixture with acooling medium passing through the tube side; and recycling means forrecycling a liquid obtained from the vertical submerged condenser to thesynthesis reactor, wherein the vertical submerged condenser is placedbelow the synthesis reactor.
 10. The revamping method according to claim9, wherein the vertical falling liquid film condenser comprises coolingmeans, and the method further comprises providing the apparatus with aline for directing a liquid at the outlet of the stripper to the coolingmeans.
 11. The apparatus according to claim 2, wherein the synthesisreactor and the vertical submerged condenser are divided from each otherand integrated with each other, and the synthesis reactor is placedabove the vertical submerged condenser.
 12. The apparatus according toclaim 11, further comprising a scrubber for scrubbing a gas that has notbeen condensed in the vertical submerged condenser, wherein the scrubberis placed in the interior of the vertical submerged condenser.
 13. Theapparatus according to claim 2, further comprising a scrubber forscrubbing a gas that has not been condensed in the vertical submergedcondenser, wherein the vertical submerged condenser and the scrubber areintegrated with each other.
 14. The apparatus according to claim 13,wherein the scrubber is placed in the interior of the vertical submergedcondenser.
 15. The apparatus according to claim 6, wherein the synthesisreactor is a horizontal type.
 16. The apparatus according to claim 13,wherein the synthesis reactor is a horizontal type.
 17. The apparatusaccording to claim 14, wherein the synthesis reactor is a horizontaltype.
 18. A method for synthesizing urea using an apparatus having asynthesis reactor for reacting ammonia with carbon dioxide to obtain aurea synthesis solution containing urea, unreacted ammonia, unreactedcarbon dioxide and water, comprising: stripping by a stripper the ureasynthesis solution with use of at least a part of raw material carbondioxide to separate a gas mixture containing the unreacted ammonia andthe unreacted carbon dioxide; condensing by a vertical submergedcondenser having a shell and tube structure, the gas mixture in anabsorbing medium on the shell side while cooling the gas mixture with acooling medium passing through the tube side; and recycling a liquidobtained from the vertical submerged condenser to the synthesis reactor,wherein the vertical submerged condenser is placed below the synthesisreactor.